Thermo-electric alloys



2,955,145 Patented Oct. 4, 1960 THERMO-ELE CTRIC ALLOYS Nils Gustav Schrewelius, Hallstahammar, Sweden, assignor to AB Kanthal, Hallstahammar, Sweden, a corporation of Sweden N Drawing. Filed .luly1'4, 1959, Ser. No. 826,916 Claims priority, application Sweden July 16, 1958 4 Claims. (Cl. "136-5) 7 This invention relates to thermo-electric alloys which are particularly suitable for use at very elevated temperatures, and which consist essentially of an intermetallic composition containing molybdenum, silicon and aluminium.

It is known (J. Appl. Phys. 24, p. 498, 1953), that MoSi within the range from -60 C. to +600 C. exhibits, with respect to platinum, a thermal which corresponds approximately to that of copper. Thus, highly temperature-resistant molybdenum disilicide can be used in a thermo-couple. It is also known (Austrian patent specification 193,632) that molybdenum disilicide with 3040% Si and various additions of other scaleresistant silicides or oxides or silicon carbide may be used in thermo-couples at elevated temperatures. Additions suggested in this patent are titanium silicide, tungsten silicide, chromiumsilicide, aluminium oxide, thorium oxide, titanium oxide, and zirconium oxide. Up to 25% of the silicon atoms of the molybdenum silicide may be replaced by carbon, boron or nitrogen.

"the thermo-electric alloys according to the present invention are distinguished from such previously known alloys in that they comprise molybdenum disilicide wherein 20-60 atomic percents of silicon have been replaced by aluminium. As a result, the crystal structure of the molybdenum disilicide, which is normally of the tetragonal C 11 lattice type, is converted entirely to the hexagonal C 40 lattice type. These lattice types are well known to those skilled in the art. (Compare Schwarzkopf et al., Refractory Hard Metals, 1953.) This conversion of the crystal structure results, surprisingly, in a considerably increased thermal for the alloys according to the present invention with respect to pure M0Si for example. Other advantages of these alloys are high mechanical strength, high oxidation resistance, and resistance to thermal shocks. By variations of the content of aluminium variations in the thermal E.M.F. can be readily obtained, and these variations are not accompanied by any significant deterioration of the mechanical or chemical properties of the alloys. In this respect, the alloys according to the present invention are distinguished from the previously known thermo-electric alloys based on molybdenum silicide.

The thermo-electric alloys according to the invention may also, if desired, comprise admixtures of other metals. Thus, up to 50 atomic percents as a maximum of molybdenum can be replaced by one or more of the metals titanium, zirconium, hafnium, tantalum, niobium, vanadium, tungsten, and chromium. The composition of the thermo-electric alloys according to the present invention may thus be expressed as wherein 0.2 x 0.6 and 0 y 0.5, and M represents one or more of the metals Ti, Zr, Hf, Ta, Nb, V, W or C1.

When the alloy contains no metal M, i.e. when y is 0,

then the alloy has the composition Mo(Si Al When a metal M is present, y preferably has a value of at least 0.05.

Thermo-couples comprising the alloys according to the invention are preferably made by a powder metallurgical process by sintering after admixture with 0.5 to 20 Wt. percent of a ceramic binding substance. Preferably, the ceramic binding substance is composed essentially of very finely powdered silica. However, it may also contain other oxides or silicon carbide. Conveniently, the final sintering is carried out in air, in which case a certain internal oxidation takes place. The ceramic component should preferably not exceed 30 percent'by weight of the material.

Thermocouples having one leg comprised of an alloy according to the invention, and another leg composed of MoSi for example, may also advantageously be used as heating resistors for producing high temperatures. In this case, the device defined by the two legs is suitably connected to serve as a thermocouple only for the short periods when the thermo-voltage is measured and to serve as an electrical heating resistor for the remaining periods. The resulting thermo-voltage may be used in practice for controlling the current supply to the resistor over a relay. Preferably the welded joint between the two legs should be disposed internally of the furnace, at or adjacent a lead-in electrode, so that it is not subjected to higher temperatures than those of the furnace room.

The following example illustrates the use of alloys according to the invention in a thermo-couple for 1600 to 1700 0:

The thermal increased regularly with the temperature and attained, inter alia, the following values:

Millivolts 800 C. 10 1000" C. 14 1200 C. l9 1400 C 24 1600 C 31 Both legs were 6 mm. cylindrical rods, made by extrusion and sintering, and joined by resistance butt welding. The most oxidation resistant negative leg may also, as an alternative, be formed as a tube which is closed at one end and surrounds the rod-shaped positive leg.

The above described leg combination may also be used as an electrical heating resistor and should then have the following dimensions: the positive leg is formed as a hair pin having one portion of 6 mm. diameter which acts as a glowing zone. One end of the loop is enlarged to 14 mm. and is long enough to extend out from the furnace as a cold lead-in electrode. The other end is Welded to a 9 mm. negative leg, which is similarly elongated to act as a cold lead-in electrode. As the alloy forming the negative leg has about half as high a specific resistance at 1600 C. as the alloy forming the positive leg, this lead-in electrode will also remain cool enough without special cooling devices. For the same reason, the welded joint will attain the same temperature as the furnace room, provided that it is disposed appropriately in the furnace, notwithstanding the fact that it is disposed adjacent the hot glowing Zone.

The alloy in accordance with the present invention described in the above example and used as the positive leg, was formed as follows; all parts being by weight; 98 parts of a metal powder made as set forth below were admixed with two parts of bentonite as a ceramic binding substance. Then this mixture was formed, dried and placedin a metal tube and sintered at 1400" C. for 30 minutes in an atmosphere of technical hydrogenl Final sintering was carriedout at 1550 C. in air for 2 minutes. The above mentionedmetal powder was of the composition (MoojjTid g)(Si0 8A1U 2 )2 and was formed in such a manner that Mo, Ti, Si and Al in theoretic quantities were heated in hydrogen gas to 1100 C. to start an exothermic reaction. The resulting sponge was then milled in a ball mill to a grain size of l-10 microns. The negative leg was formed in the same manner, using 95% MoSi and .5 bentonite.

- It'will be understood that thermocouples formed by the use of the alloys of the present invention may have' 4- in thermo-couples for elevated temperatures in oxidizing atmospheres, said alloy being of the hexagonal C 40 lattice type and having the composition (Mo M (Si 5;Al,;) Where M is at least one of the metals Ti, Zr, Hf, Nb, Ta, V, W, Cr, x has a value of 0.2 to 0.6 and y has a value of 0 to 0.5.

2. A thermo-electric alloy containing molybdenum, silicon and at least one other metal and adapted to be used in thermo-couples for elevated temperatures in oxidizing atmospheres, said alloy being ofthe hexagonal C 40 lattice type and having the composition (Mo Ti o.a o.2)z-

3. A thermo-couple having a positive leg comprising an alloy having the composition (Mo M (Si Al where M is at least one of the metals Ti, Zr, H f, Nb, Ta, V, W, Cr, x has a value of 0.2 to 0.6 and y has a value of 0 to 025, and a negative leg comprising Mosi r 4. A thermo-couple as defined in claim 3, wherein the alloy has the composition (Mo Ti )(Si Al References Cited in the file of'this'patent UNITED STATES PATENTS 2,745,928 Glasser May 5, 1955 

1. A THERMO-ELECTRIC ALLOY CONTAINING MOLYBDENUM, SILICON AND AT LEAST ONE OTHER METAL AND ADAPTED TO BE USED IN THERMO-COUPLES FOR ELEVATED TEMPERATURE IN OXIDIZING ATMOSPHERES, SAID ALLOY BEING OF THE HEXAGONAL C 40 LATTICE TYPE AND HAVING THE COMPOSITION (MO1-YMY)(SI1-XALX)2, WHERE M IS AT LEAST ONE OF THE METALS, TI, ZR, HF, NB, TA, V, W, CR, X HAS A VALUE OF 0.2 TO 0.6 AND Y HAS A VALUE OF 0 TO 0.05. 